1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the start of a synchronous motor, and especially, to an apparatus suitable for controlling the start of a synchronous motor mounted on a motor vehicle and an electric pump for controlling a working fluid of a motor vehicle driving system in which said apparatus is applied. This electric pump is, in concrete terms, an electric pump for controlling flow rate of a working fluid (oil or the like) used in a driving system mounted on a motor vehicle such as a CVT (Continuously Variable Transmission) or an automatic transmission, or in a steering system mounted on a motor vehicle such as an oil hydraulic steering apparatus.
2. Description of Prior Art
Among transmissions, such as the CVT, to be mounted on a motor vehicle, there are oil hydraulic type transmissions in which oil is used as a working fluid. For example, in Japanese Unexamined Laid-Open Patent Publication No. 2000-46 166, such a prior art of the CVT is disclosed.
This prior art has such a mechanism that, when the rotation frequency of an engine is not less than a predetermined rate, a working fluid is supplied to an automatic transmission by a mechanical pump; when the rotation frequency of the engine is less than the predetermined rate, the working fluid is supplied to the automatic transmission by an electric pump and the mechanical pump; and when the engine is stopped, the working fluid is supplied to the automatic transmission by the electric pump.
In a transmission such as a CVT, an electric pump has been conventionally used as an auxiliary means for supplying a working fluid. Further, in an oil hydraulic power steering apparatus, an electric pump is also used as an auxiliary means for supplying oil as a working fluid.
Otherwise, it may possible to supply a working fluid only by an electric pump (that is, the electric pump is not used for assisting a mechanical pump but is used as a main pump) in order to fulfill the recent energy-saving requirement of a motor vehicle.
In an electric pump for supplying or assisting to supply a working fluid used in a driving system or a steering system mounted on a motor vehicle, a brush-equipped motor has been conventionally used as its driving source. However, a driving system or a steering system mounted on a motor vehicle is disposed in an engine room or the like which is in a high temperature environment having an atmospheric temperature of not less than 120xc2x0 C. Consequently, there is a problem that the brush is worn hard and has only a short life.
Therefore, the inventors have examined the use of a brushless motor as a driving source of an electric pump. A brushless motor is a synchronous motor made to have a characteristic equivalent to that of a DC motor, and it is an apparatus including a synchronous motor and a driver for driving it.
In order to use a synchronous motor as a brushless motor, generally, a sensor for detecting the angular position of a rotor is required. An AC current in correspondence with an angular position of the rotor detected by the sensor is generated by an inverter and supplied to a field coil. The most ordinary sensor is constituted by a Hall element, but there is a problem that a Hall element has a low degree of heat tolerance and loses its function at about 120xc2x0 C. Consequently, in a synchronous motor incorporated into an electric pump, a Hall element cannot be used as a sensor.
Therefore, the inventors of the present invention have thought to use a brushless motor having no sensor as a driving source of an electric pump. Usually, a counter electromotive force is generated when a rotor rotates. Since such the a counter electromotive force has a waveform synchronous with the rotation, the angular position of the rotor can be detected by detecting the counter electromotive force. However, the inventors have encountered a problem that, at the starting time of the motor, the rotation rate of the rotor is low and the counter electromotive force is small, and therefore, the counter electromotive force is hard to detect and cannot be used as angular position signal (control signal).
In other words, a sensorless structure of a brushless motor becomes effective only after the counter electromotive force becomes so high as it can be easily detected, and after that time, the motor comes into a commutation mode (sensorless mode). In prior arts, it takes about 10 sec., or about 2 sec. if rapid, till a brushless sensorless motor comes into a commutation mode (in the case of a brushless sensorless motor incorporated into a home electric product such as an air conditioner).
However, a motor for an electric pump used in a driving system mounted on a motor vehicle is strictly required that the oil pressure of the electric pump must be built up within 0.2 sec. and the starting ability of the motor in this case is not more than 0.1 sec.
This is because, if built-up of the oil pressure for controlling an automatic transmission or non-stage transmission is slow, a delayed response of the transmission gives an operator a feeling of disorder when abruptly starting from a stopping state, at the time of a shift change or the like. For improving such a responding ability of a transmission, a method is known which comprises providing an accumulator in an oil pressure circuit and always applying a pressure higher than a predetermined value. However, in this case, an oil hydraulic apparatus becomes large in size and therefore, is not suitable to be mounted on a motor vehicle. In addition, it is preferable that the oil hydraulic apparatus not become complicated and expensive.
Another method for preventing such a delay of response may comprise preparing for abrupt starting or the like by keeping the oil pressure higher than a predetermined value even at the stopping time of the motor vehicle. However, for carrying out this method, the electric pump must be operated even at the stopping time of the motor vehicle, which is not preferable from the viewpoint of reduction of energy consumption.
The present invention has been made with such a background. And an object of the present invention is to provide a method and an apparatus for controlling starting of a synchronous motor in which the synchronous motor can start in a very short time.
Another object of the present invention is to provide a method and an apparatus for applying a synchronous motor as a brushless sensorless motor which is a driving source of an electric pump for controlling a working fluid.
A further object of the present invention is to provide an electric pump for controlling a working fluid used in a driving system mounted on a motor vehicle in which a brushless sensorless motor is a driving source.
The present invention is directed to a method and an apparatus for controlling the start of a synchronous motor having an armature (rotor) and a field coil (stationary part), comprising steps of starting the synchronous motor by supplying an AC current having a predetermined frequency to the field coil of the synchronous motor by an inverter, detecting a terminal voltage of the field coil of the synchronous motor at the time of starting, decreasing a voltage waveform component based on the AC current supplied by the inverter from the detected voltage, correcting a waveform so that a voltage waveform component based on a counter electromotive force can be confirmed, and generating a control signal to be supplied to the inverter using the corrected voltage waveform.
The waveform correcting means can use a circuit for decreasing influence of the voltage waveform by decreasing the voltage waveform component based on the AC current supplied by the inverter from the detected voltage which is a composite waveform consisting the voltage waveform based on the AC current supplied by the inverter and the voltage waveform based on the counter electromotive force of the synchronous motor.
The waveform correcting means includes a voltage dividing circuit having at least two series circuits of resistive elements each connected to a terminal of the synchronous motor and an additional resistive element selectively connected in parallel to one of the resistive elements of the voltage dividing circuit, and the additional resistive element has such a structure that its connection/disconnection is changed over in correspondence with the output waveform of the inverter.
The present invention is further directed to an electric pump for controlling a working fluid used in a driving system or a steering system of a motor vehicle, which includes a pump body for transferring the working fluid, a synchronous motor for rotating the pump body and a driver of the synchronous motor, and the apparatus for controlling starting is adopted in said driver of the synchronous motor.
According to the present invention, though a counter electromotive force is small at the starting time of the synchronous motor, a structure is provided so that the small counter electromotive force can be surely detected. That is, as shown in FIG. 1A, at the starting time of the synchronous motor, an AC current of a low frequency is supplied to the field coil from the inverter, and the armature (rotor) is withdrawn and begins to rotate. When the armature rotates, a counter electromotive force is generated, and the amplitude of the counter electromotive force becomes large with the rise of the rotation of the armature. The terminal voltage of the field coil is a composite waveform consisting a voltage waveform based on an AC current supplied by the inverter and a voltage waveform based on the counter electromotive force (FIG. 1B).
Generally, this waveform is integrated in an integrating circuit and used for phase comparison. However, it is disadvantageous that, when the component based on the counter electromotive force is small, the counter electromotive force component has little influence on the integrated waveform if the composite waveform is integrated.
That is, the phase of the voltage based on the counter electromotive force leads the phase of the waveform based on the applied AC current, but the phase lead of the counter electromotive force cannot be detected.
Consequently, according to the present invention, since the waveform based on the AC current supplied by the inverter is a waveform outputted by the inverter itself and therefore, is already known, such a structure is invented that, by eliminating said waveform from the composite waveform, the counter electromotive force component becomes apparent.
With such the structure, the waveform given to the integrating circuit is as shown in FIG. 1C, and this is a waveform in which the waveform based on the counter electromotive force becomes apparent.
Therefore, even when the rotation rate of the armature (rotor) is low at the starting time of the synchronous motor, the phase difference (phase lead) of the component based on the counter electromotive force can be reflected on the control signal supplied to the inverter, and the synchronous motor can come into a commutation mode (sensorless mode) in a short time immediately after the starting.
As a result, according to the present invention, there can be provided a method and an apparatus for controlling starting of a synchronous motor having an excellent starting ability and capable of coming into a commutation mode in a short time.
Further, according to the present invention, there can be provided a circuit for controlling the start of a synchronous motor suitable as a driving source of an electric pump used in a driving system, a steering system or the like.
Further, according to the present invention, there can be provided an electric pump for use in a driving system, a steering system or the like, which is inexpensive and has an excellent starting ability.
By using the electric pump according to the present invention, the oil pressure reaches a predetermined value in a short time after starting of the pump, and therefore a transmission smoothly operates without delay of response even at the time of, e.g., abrupt starting or shift change, thus preventing an operator from receiving a feeling of disorder.